1. Field of the Invention
The present invention relates to an apparatus for controlling a deflectable head for use with a VTR (video tape recorder), and more particularly is directed to an apparatus and method for reducing a secular or slowly occurring variation of a bimorph distortion detection device attached to a bimorph leaf mounting a deflectable head of a VTR.
2. Description of the Prior Art
A VTR is known in which a magnetic head chip is attached to one end of a bimorph leaf which has its other end fixed to a rotary drum body and a drive signal is supplied to the bimorph leaf so that even in a variable speed playback mode, the rotary magnetic head chip can be made to positively scan a skewed or slant track formed on a magnetic tape. For instance, a bimorph head tracking system using the so-called wobbling technique is disclosed in U.S. Pat. No. 4,229,773. Also, a technique using a strain gauge to detect the deflection of a bimorph leaf is disclosed in U.S. Pat. No. 4,172,265 and a technique using a damping A.C. signal to correct the hysteresis of a bimorph leaf is disclosed in U.S. Pat. No. 4,513,334.
In a bimorph driving apparatus for such bimorph deflectable head, the bimorph leaf contains in its electro-mechanical transducer characteristic a hysteresis characteristic and, the secular or slowing occurring variation of its sensitivity and the variation of its sensitivity in response to temperature changes are large. Therefore, in a correcting circuit of the prior art, strain gauges are attached to both sides of the bimorph leaf and a distortion or deflection detecting output derived from the strain gauges is negatively fed back to the driving signal.
However, such prior art correcting circuit for a bimorph deflectable head has the following defects. The offset amount of the distortion or deflection detecting output from the strain gauges attached to the bimorph leaf has a secular variation so that there is an error occurs in the indication of the distortion, displacement or deflection amount of the bimorph deflectable head. In other words, after a damping A.C. signal is applied to the bimorph leaf, the deflection or distortion of the bimorph leaf becomes zero. Accordingly, the signal output from the strain gauge (e.g., in practice, signal generated by the change of the resistance value of the strain gauge) should become a constant value corresponding to the zero distortion or deflection of the bimorph leaf. However, if in the strain gauge itself there is a scattering or a secular variation occurs, the signal output from the strain gauge can take various values relative to the zero distortion or deflection of the bimorph leaf and frequently does not become the constant value. As a result, the distortion or deflection of the bimorph deflectable head can not be controlled correctly.